What is the difference between Sobol and Correlated Multi-Jitter sampling patterns?
In 2013 there was a long debate in a blenderartists thread, where some argued that for some scenes Correlated Multi-Jitter is better than Sobol, while others saw no real difference. 

Cycles creator Brecht van Lommel was initially skeptic, he wrote:


  These correlated multi-jitter vs. Sobol comparisons so far seem to confirm my own findings, which is that neither sampler is consistently better or particularly different even (...). Conclusion seems to be that the sampler sequence we have now is about as good as it's going to get, though it's nice to have a second good quality sampler implemented in the code as a sanity check for me. 


But later he was convinced, and wrote:


  Ah, I missed those comparisons. It seems indeed correlated multi-jitter is handling BSDF x mesh light source sampling better. Maybe that's something that can be fixed in Sobol, need to investigate. 


One of the scenes where Correlated Multi-Jitter was better, was a diamond scene from BlendSwap. I wanted to see if the Sobol improved since 2013, so now (December 2016) I repeated the test with Blender 2.78a, but I still see a large advantage for Correlated Multi-Jitter.

100 samples, Sobol:

100 samples, Correlated Multi-Jitter:


The rendering time with Correlated Multi-Jitter was 6% longer than with Sobol, but there is considerably less noise in the image. There are some  fireflies, but they should be easy to filter in post, because their color so drastically differs from the surrounding pixels. Here's the Correlated Multi-Jitter post-processed with a very simple image filter that only removes single-pixel noise:


On the other hand I saw no examples where Sobol was considerably better than Correlated Multi-Jitter, so in general it might be a good idea to change the default setting.

Also note that there is some work going on in the sampling area, for example Blue noised dithered sampling and Micro jitttered sampling. These are planned for 2017.